In physics class, the girl to boy ratio is $3$ to $4$. If there are $15$ girls, how many boys are there?
Explanation: A ratio of $3$ girls to $4$ boys means that there is a group of $3$ girls for every group of $4$ boys. If there are $15$ girls, then there are $5$ groups of $3$ girls each. Because we know the ratio is $3:4$ , there must be $5$ groups of $4$ boys each. There is a total of $20$ boys in physics class.